The present invention relates to the field of digital storage devices, and, more particularly, to controlling the position of a beam incident on a track of a rotationally mobile carrier. Moreover, the invention relates to the control and the determination of a position error of the incident beam with respect to the track.
The invention may advantageously (but not exclusively) be applied to digital discs, in particular to multifunction digital discs such as Digital Versatile Discs (DVDs), for example, for storing image data in a compressed manner.
A digital disc includes a single spiral track whose relief is representative of binary information stored on the track of the disc. The track of the disc is illuminated by an incident optical beam (e.g., a laser spot) and several photodetectors (e.g., four) detect the reflections of the light beam on the disc. The optical pick-up formed by the photodetectors then delivers four elementary signals each provided by a respective photodetector. It also delivers an overall or useful signal that is equal to the sum of the four elementary signals. The binary information read on the track is extracted from the useful signal.
Slaving of the optical beam to the track of the rotationally mobile disc is performed exclusively on the basis of the four elementary signals delivered by the photodetectors. More precisely, the signals are summed in pairs to form two signals which are equalized in an analog equalizer before being shaped, by comparison with a threshold, in two comparators. The two signals thus shaped are mutually phase shifted if the laser spot is not situated on the track. The phase difference between these two signals is then detected, and this phase difference is in fact a mutual time gap between these two signals. The time gap corresponds to the positioning error of the beam with respect to the track. This positioning error is then conventionally used in a servocontrol loop to modify the incident optical system and slave the optical beam back to the track.
Such a prior art device includes a considerable number of analog components which may be relatively bulky. Moreover, as technology advances, the modification and production of new components of the device require considerable design and production time. Additionally, when a track jump instruction is received there may be an abrupt movement of the incident beam from one portion of the track situated somewhere on the disc to another portion of the track situated elsewhere on the disc. This may occur, for example, where a portion is situated further outboard or further inboard, and results in the slaving being released. Also, between the time of instruction and the moment at which the carriage bearing the optical system actually starts, one no longer knows the direction in which the track portions are overstepped. This is especially true if the disc exhibits an eccentricity.
To remedy this, a signal known in the art as a xe2x80x9cmirror signalxe2x80x9d may be calculated from the lower amplitude of the useful signal. More precisely, the bottom envelope of the useful signal is compared with a threshold to yield the mirror signal. Also, the mirror signal thus obtained is always in phase quadrature with respect to a binary signal representing overstepping of a portion of the track. A determination of the sign of the phase difference between the mirror signal and the track overstepping signal yields the direction of overstepping of the track. Even so, it is difficult to perform a large number of processings on these two signals, in particular filterings, due to the risk of disturbing the phase relationship. Moreover, it is also difficult to effectively fix the threshold of comparison of the bottom envelope of the useful signal.
An object of the invention is to improve accuracy in determining the positioning error while reducing the need for devices such as analog equalizers.
Another object of the invention is to determine the direction of overstepping of a portion of the track without requiring the creation of a mirror signal.
These and other objects, features, and advantages in accordance with the present invention are provided by a method for controlling the position of an optical beam incident on a track of a mobile carrier of information which in particular uses a cross-correlation function between two mutually phase-shifted sampled signals. The search for the maximum of the cross-correlation function will yield, at each current instant, the current value of the phase shift between the two signals. That is, it yields the current value of the time gap between these two signals.
More precisely, the method is for controlling the position of an optical beam incident on a track of a rotationally mobile carrier of information in which the beam reflected by the disc is picked up by an optical pick-up. The optional pick-up may include several photodetectors respectively delivering several elementary signals. The positioning error of the beam with respect to the track is determined from the elementary signals.
Two sampled signals (hereinafter xe2x80x9csecondary signalsxe2x80x9d) have a mutual time gap representative of the positioning error of the beam with respect to the track. The secondary signals and the successive current values of the mutual time gap are determined at the sampling frequency by searching, at the sampling frequency, for the successive current maximum of the cross-correlation function between the two sampled secondary signals.
The method according to the invention is an essentially arithmetic optimum process which leads to improved performance and which is also less sensitive to noise. Moreover, the cross-correlation function operates on the entirety of the sampled signals and not merely on the transitions of these sampled signals (where a transition is the overstepping by a predetermined threshold (e.g., the zero value) of the sampled signal), as previously described in French patent application no. 9903237 assigned to the present assignee. The process according to the invention therefore also alleviates the problem of estimating the temporal instants of these transitions.
Each current maximum of the cross-correlation function may be searched for on the basis of a set of samples of this cross-correlation function corresponding respectively to a predetermined set of values of reference time gaps. Stated otherwise, the integration of the cross-correlation function is restricted to a window. The value of the time gap corresponding to the current maximum may be determined, and this value yields the current value of the mutual time gap between the two sampled secondary signals.
Each reference time gap may advantageously be an integer multiple of a base time gap whose value depends on the frequency band occupied by the two sampled secondary signals. Moreover, one of the reference time gaps may be zero and the others pairwise equal and of opposite sign.
The frequency band occupied by the two secondary signals may be proportional to the speed of rotation of the portion of the track read. Thus, this base time gap is not the same for speeds of 1xc3x97 and 16xc3x97 (where a speed of rotation of 1xc3x97 corresponds to 4 m/s). In general, the choice of the value of this base time gap is not critical to the invention, but an inappropriate value may lead to degraded performance. A person skilled in the art will be able to adjust the value of this base time gap, as well as the number of samples of the cross-correlation function, to cover all the possible time gaps between the two sampled signals and also to obtain a cross-correlation function which is neither under- nor over-sampled.
The number of reference time gaps may be equal to five, and the base time gap chosen may be advantageously less than the inverse of the width of the frequency band. The determination of the value of the time gap corresponding to a current maximum may include selecting, from among the samples of the cross-correlation function, of that one having the largest value and of the two samples flanking it. This determination also includes a parabolic interpolation between the three selected values and the calculation of the ratio between two factors resulting from this parabolic interpolation.
It should be noted here that using parabolic interpolation requires a minimum of three values of the cross-correlation function. This being so, in the case where only three values of the cross-correlation function are used (corresponding to a set of three reference time gaps), a value of the base time gap is preferably chosen which is large enough to determine the maximum of this cross-correlation function. In the case where five samples are used, the value of the base time gap may be smaller.
A first binary output signal representative of the possible overstepping of the track by the incident signal is thus formulated on the basis of the numerators of the successive ratios. A second binary output signal representative of the direction of overstepping of the track by the incident signal is formulated on the basis of the numerators and of the denominators of the successive ratios.
A device for controlling the position of an optical beam incident on a track of a rotationally mobile carrier according to the invention includes a pick-up able to pick up the beam reflected by the disc. The pick-up may include several photodetectors. The device also includes a controller or control means able to determine the positioning error of the beam with respect to the track from the elementary signals respectively delivered by the photodetectors.
More specifically, the control means may include a preprocessor or preprocessing means able to formulate, from the elementary signals, two secondary signals sampled by a sampling clock signal. A mutual time gap between the two sampled secondary signals may be representative of the positioning error of the beam with respect to the track. Furthermore, the control means also include a cross-correlator or cross-correlation means able to perform, at the signal sampling frequency, the cross-correlation function between the two sampled secondary signals, and a processor or processing means able to determine the successive current values of the mutual time gap. This may be done by searching, at the sampling frequency, for the successive current maximums of the cross-correlation function between the two sampled secondary signals.
Furthermore, the cross-correlation means may include two inputs for respectively receiving the two sampled secondary signals and a set of multipliers connected to the two inputs by a delay circuit or means and respectively delivering a set of sampled intermediate signals. The set of sampled intermediate signals respectively corresponding to the products of the temporally mutually shifted sampled secondary signals multiplied by reference time gaps. The cross-correlation means may further include a set of low-pass filters respectively connected to the outputs of the various multipliers and a storage device or means connected to the outputs of the low-pass filters and able to store the successive sets of samples of the cross-correlation function respectively associated with the various reference time gaps.
The processing means may thus search for each current maximum of the cross-correlation function based upon each set of samples stored in the storage means and to determine the value of the time gap corresponding to this current maximum. This value yields the current value of the time gap between the sampled secondary signals.
The processing means may include a selector or selection means able to select, from among the samples stored in the storage means, a sample having the largest value and the two samples flanking it. The processing means may also include an interpolater or interpolation means able to perform a parabolic interpolation between the three values selected by the selection means. The processing means may also calculate the ratio between two factors resulting from this parabolic interpolation. This ratio yields the current value of the mutual time gap between the two sampled secondary signals.
The device according to the invention may also advantageously include a post-processor or post-processing means able to formulate, from the numerators of the successive ratios, a first binary output signal representative of the possible overstepping of the track by the incident signal. It may also formulate, from the numerators and from the denominators of the successive ratios, a second binary output signal representative of the direction of overstepping of the track by the incident signal.
A multifunction digital disc reader according to the present invention including a control device as defined above is also provided.